We can trace our evolutionary heritage back millions of years, but it's surprisingly tricky to say with certainty which hominid species came directly before us. Now it looks like one of our evolutionary cousins is actually our parent species.

When we say "parent species", what we really mean is the most recent common ancestor of humans and our closest relatives, Neanderthals. Because the differences between various extinct hominids can be very subtle, we can't necessarily be sure how precisely all the various species slot into our family tree. That might be about to change, thanks to new research that says Homo heidelbergensis is the common ancestor of humans and Neanderthals.

Researchers at France's National Center of Scientific Research (CNRS) focused on a particular fossil, a 400,000 year old partial cranium discovered in Ceprano, Italy. Because it initially appeared to be a new hominid species, it was given a scientific name after the town, Homo cepranensis. But this study, which compared the fossil with the skulls of 68 modern humans as well as 42 other fossils that ranged from 1.8 million to just 12,00 years old, found that the Ceprano skull was actually an archaic member of Homo heidelbergensis.

What this study really illuminated was the remarkable geographic range of Homo heidelbergensis, with the Ceprano skull sharing crucial similarities with fossils found throughout Africa and Eurasia over a time period spanning from 781,000 to 126,000 years ago. The idea is that, aided by the favorable climates of the Middle Pleistocene starting around 780,000 years ago, Homo heidelbergensis spread far and wide throughout the Old World.

Around 400,000 years ago, this mobility began to decrease and Homo heidelbergensis became more isolated, paving the way for the clear emergence of Neanderthals and modern humans in Eurasia and Africa respectively. Although Homo heidelbergensis has generally been seen as a precursor to Neanderthals, its precise relation to us has been less clear. This fossil analysis is some of the most compelling evidence yet that we can in fact trace an evolutionary lineage back through Homo heidelbergensis.

Paleontologist Chris Stringer of the Natural History Museum in London says he generally agrees with the findings of the CNRS team, but he stresses that there's still plenty that we don't know:

"I have long argued that Homo heidelbergensis represented our common ancestor with the Neanderthals about 400,000 years ago, and the Ceprano fossil, with its newly-determined late date, is well-situated chronologically to be part of this common ancestral group. However, it is quite a primitive specimen in several respects and therefore it may be that, like some other samples of heidelbergensis in Africa and Europe, it does not represent the actual last ancestral population. In my view, we still do not know where that particular population existed, and it may even have lived in a place from which we have very little evidence at present, such as western Asia."